This invention relates to a control circuit for an electro-magnetically operated contactor which comprises one or more pairs of contacts and a solenoid which is energized by direct current to cause closure of the contacts.
Such contactors are designed to have a nominal working voltage for the solenoid coil, but will pull in at a somewhat lower applied voltage. If the voltage applied to the coil is reduced after the contactor has pulled in, the contacts will remain closed until the applied voltage reaches a drop-out voltage which is appreciably lower than the pull-in voltage.
Such a contactor may be used to switch the supply from a battery to an electric motor, for example a motor for driving a hydraulic pump in a battery-operated vehicle, such as a fork-lift truck.
Many electrically powered fork-lift trucks have hydraulic systems for raising and lowering the forks and for tilting the masts relative to the supporting structure. The hydraulic fluid pressure is created by such hydraulic pump. The driving motor is started and stopped by closing and opening microswitches which are controlled with the opening and closing of a hydraulic control valve. These switches energize the contactor. The hydraulic control valve normally has a spring centering arrangement for returning the valve spool to the neutral position when the control lever is released from an operating position.
One of the problems encountered with this arrangement is that the control valves have a tendency to overshoot sufficiently to cause the switch contacts, and hence the power contactor, to close for a split second and then open again. D.C. motors typically draw a high peak current during initial starting, and the re-opening of the contactor occurs during a time when the peak current is passing through the contactor contacts. This causes intense arcing, resulting in rapid burning of the contacts. Rapid opening and closing of the switch contacts is also frequently initiated by the truck driver by operating the control valve with quick, short lever actuations for moving the fork or mast in small increments for precise positioning.
A further problem is that as the battery becomes discharged during normal use of the vehicle, the battery terminal voltage decreases, and when a heavy current demand is applied to the battery, for example on starting the motor under heavy load conditions, the terminal voltage can fall to a very low value.
Since the contactor coil supply is also taken from the battery, this low terminal voltage on heavy load may be insufficient to hold in the contactor. Hence, when the load is applied, the contactor will begin to drop out, but as the contacts open, the load will be removed from the battery and the terminal voltage will immediately rise. This will cause the contactor to pull in again, once again applying the load to the battery. This action will be repeated continuously. If the contacts continue to open and close on the heavy direct current load in this manner, either of two extremely serious situations can arise. The contacts may weld together, in which case the motor will become uncontrollable. On the other hand, if they continue to open and close cyclically, the contacts will burn away due to the arcing which takes place, and destruction of the contactor can result.
It is desirable to avoid undesirable opening and closing of the contacts of a d.c. operated electromagnetic contactor.